This invention concerns precursors for use in chemical vapour deposition techniques the production of electro-ceramic devices therefrom, and their use in ferro-electric memories and I.R. detectors.
Metalorganic chemical vapour deposition (MOCVD) is a preferred method for depositing thin films, i.e. in the order of a few Am of ferroelectric metal oxides, such as lead zirconate titanate, [Pb(Zr,Ti)O3 or PZT and lanthanum-modified lead zirconate titanate. [(Pb,La)(Zr,Ti)O3 or PLZT]. These electro-ceramic materials have a wide range of useful dielectric, ferroelectric, piezoelectric, pyroelectric and electrostrictive properties, giving rise to a variety of potential applications ranging from thermal imaging and security systems to integrated optics and computer memories, e.g. DRAMS and non-volatile FERAMS.
The MOCVD technique involves transporting a metal as a volatile metalorganic compound in the vapour phase followed by thermal decomposition usually in the presence of oxygen on an appropriate substrate. The different types of substrate can be divided into three groups, namely oxides, semiconductors and metals. Examples of suitable oxide substrates are SiO2, SrTiO3, MgO and Al2O3. Semiconductor substrates include silicon (Si) and germanium (Ge) and metal substrates may be, for example, molybdenum (Mo) or tungsten (W). MOCVD has a number of advantages over other deposition techniques, such as sol-gel or physical vapour deposition. MOCMD offers potential for large-area deposition, excellent film uniformity and composition control, high film densities and deposition rates and excellent conformal step coverage at dimensions less than 2 xcexcm. Furthermore, MOCVD processes are compatible with existing silicon chemical vapour deposition processes used in ULSI and VLSI applications.
Precursors for MOCVD of electro-ceramic the films are generally metal xcex2-diketonates, such as, for example, lead bis-tetramethylheptanedionate (Pb(thd)2, or metal alkoxides. WO 96/40690 discloses various metalorganic complexes of the formula MAyX, wherein M is a y-valent metal, A is a monodentrate or multidentrate organic ligand coordinated to M which allows complexing of MAy with X, y is a integer having a value of 2,3 or 4 and X is a monodentrate, or multidentrate ligand coordinated to M and containing one or more atoms independently selected from C, N, H, S, O and F. A may be a xcex2-diketonate and X may be tetraglyme, tetrahyrofuran, bipyridine, crown ether or thioether.
It is important that the precursors are volatile enough to be transported efficiently at source temperatures which are below the precursor decomposition temperature. In other words, there should be an adequate temperature window between vaporisation and decomposition. The precursors used need to be compatible and not pre-react. They should decompose to form the desired metal oxide in the same temperature region. Ideally, precursors have low toxicity and are stable under ambient conditions.
Available metal alkoxide and metal xcex2-diketonate precursors generally have only very low vapour pressures, so that high source temperatures are required for MOCVD. For example, Pb(thd)2 is typically transported at above 130xc2x0 C. and Zr(thd)4 at above 166xc2x0 C. In conventional MOCVD in which a carrier gas is passed through a precursor held at a high temperature for the duration of the deposition process, this can lead to thermal ageing, i.e. decomposition of the precursor prior to transport into the reactor.
One way of avoiding this problem has been to use liquid injection MOCVD, in which a solution of the precursor(s) in an appropriate solvent, e.g. tetrahydrofuran, is evaporated and then transported to the substrate. In this way the precursor is only subjected to heating during evaporation rather than for the duration of the MOCVD process.
For ease of handling and volatility, toxicity and decomposition characteristics, the optimum precursor combination for MOCVD of PZT is Pb(thd)2, Zr(thd)4 and either Ti(OPri)4 or Ti(OPri)2(thd)2. However, there is a problem with using Zr(thd)4, in that it is too stable, making it difficult to control the stoichiometry of PZT during liquid delivery MOCVD. In particular, there is a large difference between the decomposition temperature of Zr(thd)4 and the most useful lead precursor Pb(thd)2. This results in a significant difference between the temperatures for diffusion (or mass-limited) oxide film growth between the two precursors and the need to use high substrate temperatures to decompose the Zr(thd), source leads to a loss of lead from the PZT films by evaporation.
Zirconium alkoxides, such as Zr(OPri)4 and Zr(OBut)4 are predicted to be much less thermally stable than Zr(thd)4 but are highly air and moisture sensitive making them difficult to manufacture in pure form and too unstable for long term storage.
An object of this invention is to provide alternative Zr precursors for use in MOCVD, especially for depositing PZT and PZLT.
Another object of the invention is to provide an improved method of depositing zirconium containing metal oxides in thin films.
According to a first aspect of this invention there is provided a zirconium precursor suitable for use in MOCVD having the formula
Zrx(OR)yLz
wherein R is an alkyl group
L is a xcex2-diketonate group.
x=1 or 2
y=2, 4 or 6, and
z=or 2
According to a second aspect of the invention there is provided a method of depositing thin films of or containing zirconium oxide using metalorganic precursors in an MOCVD technique, wherein the zirconium precursor has the formula
Zrx(OR)yLz
wherein R is an alkyl group
L is a xcex2-diketonate group.
x=1 or 2
y=2, 4 or 6, and
z=1 or 2
The preferred alkyl groups R are branched chain alkyl groups, preferably having less than 10 carbon atoms, more preferably having 1 to 6 carbon atoms, especially iso-propyl and tertiary-butyl groups.
The preferred xcex2-diketonate groups L include those of the general formula 
wherein R1 and R2 are the same or different and are straight or branched, optionally substituted, alkyl groups or, optionally substituted, phenyl groups. Examples of suitable substituents include chlorine, fluorine and methoxy.
Examples of suitable xcex2-diketonate groups for use in precursors of the invention include the following:
In one preferred embodiment of the invention the zirconium precursor has the following formula:
Zr(OR)2L2
wherein R and L are as defined above.
Typical examples of such zirconium precursors include Zr(OPri)2 (thd)2 and Zr(OBut)2(thd)2 
These compounds are believed to be particularly suitable for use in the method according to the invention, especially in liquid injection MOCVD.
In another preferred embodiment of the invention, the zirconium precursor has the following formula:
Zr2(OPri)6(thd)2
Again this compound is believed to be particularly suitable for use in the method of the invention, especially in liquid injection MOCVD.
Compounds of the invention may be produced by reaction of an appropriate zirconium alkoxide with an appropriate xcex2-diketone.
The method of the invention is particularly useful for depositing on a substrate thin films, i.e. in the order of up to 5 xcexcm of lead zirconate titanate (PZT) using a zirconium precursor according to the invention with a lead precursor, such as Pb(thd)2 or lanthanum-modified lead zirconate titanate (PLZT). Typical substrates include SiO2, Si, SrTiO3, MgO, Al2O3, Ge, Mo and W.
According to a further aspect of the present invention there is provided a method of forming an electro-ceramic device comprising the steps of depositing a lower conducting electrode onto a substrate, depositing a film layer of or containing zirconium oxide onto said electrode and depositing an upper or further conducting electrode thereon, wherein the zirconium oxide layer is formed from the zirconium precursor having the formula:
Zrx(OR)yLz
wherein R is an alkyl group;
L is a xcex2-diketonate group;
x=1 or 2;
y=2, 4 or 6; and
z=1 or 2.
The lower conducting electrode and upper conducting electrode is preferably a metal, for example, platinum. The substrate is preferably a silicon wafer or circuit. An electro-ceramic device formed by this method is particularly suitable for use in ferro-electric memories and infra-red detectors.